A commercially available process for removing sulfur dioxide from flue gases uses an aqueous slurry of lime, or other calcium component, preferably enhanced with magnesium, to scrub a countercurrent flow of the flue gases in a wet scrubbing unit. The lime, in removing sulfur dioxide from the flue gases, produces calcium sulfite, which is removed from the scrubbing system as a bleed stream of the scrubbing unit effluent. The bleed stream is passed to a clarifier or thickener and a thickened slurry or sludge separated from the aqueous media. The aqueous media is returned to the scrubber unit as a recycle stream, while the sludge is discharged from the scrubbing system and discarded, after further concentration, if desired. A problem that arises is the poor dewatering properties of the calcium sulfite sludge removed from the scrubbing system.
The poor dewatering properties of calcium sulfite sludges from lime scrubbing systems adds to the expense of operating such systems. Currently, only very limited improvements can be made, such as by the addition of sulfur, or thiosulfate, or polymeric floculants to the system so as to improve the dewatering characteristics of the sludge. Another approach is to lower the solids content of the slurry to minimize secondary nucleation, but such is not feasible because of limitations owing to the design of the scrubbing system.
In magnesium-enhanced lime scrubbing processes, because of the high solubility of magnesium sulfite and its tendency to form ion pairs in solution, sulfite, not bisulfite, is elevated, while calcium is suppressed by adequate concentration of magnesium. Therefore, the scrubbing system is rich in alkalinity and gypsum scale does not form.
However, this process is known for producing a sludge that is difficult to dewater without applying forced oxidation, the sludge consists primarily of calcium sulfite and some coprecipitated calcium sulfate. It has been speculated by J. Chang and T. Brna, "Evaluation of Solids Dewatering for a Pilot-Scale Thiosorbic Lime SO.sub.2 Scrubber, AIChE 1987 National Meeting, Houston, Tex. (April 1987), that magnesium causes deterioration of the dewatering properties of flue gas desulfurization (FGD) solid products. It was demonstrated by P. Tseng and G. Rochell, "Calcium Sulfite Hemihydrate: Crystal Growth Rate and Crystal Habit," Environmental Progress, 5 (1) pp 5-11 (1986), that a calcium sulfite hemihydrate crystal defect, caused by coprecipitated calcium sulfate, adversely affected the crystal habit and therefore settling and dewatering properties of the sludge. Low solids content was also suggested by F. Bazek et al. in "Effect of Wet Lime FGD Operating Conditions on Improving Particle Size and Dewatering of Sludge", Tenth Symposium on Flue Gas Desulfurization, November 17-21 (1986) and by L. Benson et al., "Improving Sludge Dewatering in Magnesium-Enhanced Lime FGD Systems", Proceedings of the EPA/EPRI First Combined FGD and Dry SO.sub.2 Control Symposium, St. Louis, Mo., October (1988), to minimize secondary nucleation and therefore enhance average particle size. In addition, a double-drawoff crystallizer configuration, separating large particles from fine nuclei and sending only large particles to thickeners, seems promising to improve sludge dewatering properties, as suggested by J. Chang and T. Brna, "Gypsum Crystallization for Limestone FGD", Chemical Engineering Progress, 82:51 (1986).
Although intensive research activities have been conducted to improve sludge dewatering properties, the present inventors believe there are still some unexplored areas, for example, controlling nucleation at the dissolving lime surface by modifying the surface chemistry of lime or by modifying the way lime is introduced to the FGD system.
It is an object of the present invention to improve the dewatering properties of thickener underflow sludges from a lime scrubbing system and thus reduce the operating costs of such a system.
It is another object of the present invention to reduce the primary nucleation of calcium sulfite in a wet scrubbing system and increase the average particle size of the calcium sulfite, and make the dewatering of aqueous sludges from such a system easier to effect.